1. Field of the Invention
This invention relates to a molding material suitable as an optical material, and more specifically to a molding material giving a small birefringence and being free from occurrence of voids and silver streaks when used to form an optical transparent substrate by injection molding. This invention also relates to an optical recording medium formed of said material.
2. Related Art
Optical transparent substrates such as an optical disk are shaped generally by injection molding. Such substrates are featured by comparatively small thicknesses such as 1.2 mm but large diameters such as 120 to 300 mm, and require so-called thin-wall precision molding. It is further required to accurately replicate pits and grooves which are as fine as several micrometers.
As a material for that use, hydrogenated products of norbornene-type monomers such as tetracyclododecene and dicyclopentadiene attract attention (JP-A-60-26024, JP-A-61-24826, JP-A-63-317520, EP 303,246 and EP 317,262). The reasons therefor are that these hydrogenated products have excellent transparency and heat resistance, low water absorptivity and low birefringence as well as excellent moldability such as high fluidity and high mold releasability.
However, these hydrogenated products are unsatisfactory as compared with polymethyl methacrylate, although they have much lower birefringence than a polycarbonate. Further, these hydrogenated products have problems that fine voids occur within a substrate formed by injection molding, that a phenomenon causing haze called flash takes place on a substrate surface, and that silver lines called silver streaks are caused.
The present inventors have therefore studied to overcome these problems, and found that these hydrogenated products have varied birefringences depending upon molecular weight and molecular weight distribution, and that low volatiles such as an unreacted monomer, solvent, etc., are contained in conventional hydrogenated products and cause voids, flash, etc. Further, some of antioxidants incorporated into the hydrogenated products have been found to promote the occurrence of voids and silver streaks.